Artemisinin, an example of sesquiterpenes, has received attention as a compound having antimalarial activity.
Isoprenoid compound refers collectively to compounds biosynthesized from isopentenyl diphosphate (IPP) as a basic component. IPP is biosynthesized via the mevalonate pathway (MVA pathway) or the non-mevalonate pathway (MEP pathway). The sesquiterpene artemisinin is considered to be mainly biosynthesized from IPP derived from the MVA pathway in the cytoplasm via the intermediate metabolite farnesyl diphosphate (FPP).
The first reaction step of the artemisinin metabolic pathway is the biosynthesis of amorpha-4,11-diene by amorphadiene synthase (ADS) from FPP, an intermediate metabolite common to metabolic pathways of other compounds. Accordingly, increasing amorpha-4,11-diene biosynthesis is very beneficial for increasing artemisinin production and there is room for improvement by using metabolic engineering techniques.
Natural rubber, an example of polyisoprenoids, currently used in industrial rubber products is produced by growing Hevea brasiliensis (Para rubber tree) of the family Euphorbiaceae and collecting latex therefrom. Hevea brasiliensis has laticifer cells that biosynthesize and accumulate natural rubber (polyisoprenoid).
Although used in a wide variety of applications in large quantities as a raw material of rubber products, natural rubber is being collected substantially only from Hevea brasiliensis. 
Hevea brasiliensis, however, is a plant that can grow only in limited areas such as in Southeast Asia or South America. Moreover, Hevea brasiliensis requires about seven years after planting to mature enough for rubber extraction, and the period during which natural rubber (isoprenoid) can be extracted is limited to 20 to 30 years.
As the demand for natural rubber is expected to rise in the future mainly in developing countries, there are concerns about the depletion of natural rubber resources. However, for the reason mentioned above, it is difficult to greatly increase production of natural rubber by planting Hevea brasiliensis as before. Thus, there is a need for a new method for increasing production of natural rubber.
For example, the following methods for improving the efficiency of natural rubber production in rubber-producing plants are known: ethylene or ethephon (2-chloroethylphosphonic acid) is applied to the trunk of a rubber-producing plant to prevent latex exuding from the latex vessels from coagulating at the cuts, thereby increasing the efficiency for collecting latex; and lanolin containing, for example, jasmonic acid or its precursor, linolenic acid, is applied to the trunk of a rubber-producing plant to promote laticifer differentiation, thereby increasing the density of latex vessels (Non-Patent Literature 1). These methods, however, do not directly act on the mechanism of natural rubber production and thus have limited effects on production increase.